1. Field of the Invention
The present invention relates to a beam accelerator for generating high-energy charged particle beams or high-energy X rays used in cancer treatment, sterilizing and the like, and in particular, relates to an FFAG-type, circular, magnetic induction (betatron) accelerating beam accelerator which uses a fixed magnetic field to deflect charged particle beams.
2. Description of the Related Art
Beam accelerators accelerate charged particles such as electrons and the like. These accelerated charged particles irradiate an X ray conversion target of copper, tungsten, and the like to generate X rays, and cancer treatment, sterilizing and the like is performed by irradiating affected areas with the X-rays. The beam accelerator of the present invention is an FFAG (Fixed Field Alternating Gradient) accelerator using a fixed magnetic field to deflect charged particle beams, and has a small size and a high output. The only extant sample of an electron accelerating FFAG beam accelerator is the MURA (Midwestern Universities Research Association) prototype in the United States (for example, see Non-patent Publication 1)
Output voltage limiting conditions of conventional FFAG beam accelerators will be described. When an electron beam current is increased, efficient acceleration becomes problematic because the electron beam diverges in a region where it cannot be accelerated sufficiently. In order to control this divergence, accelerating voltage may be increased and acceleration performed at an earlier point in time to make a high energy beam prior to divergence. That is, the accelerating voltage may be increased proportional to the time-variance of the magnetic flux. In order to do this, the exciting frequency applied to the accelerator core must be increased.
F. T. Cole et al., THE REVIEW OF SCIENTIFIC INSTRUMENTS, volume 28, number 6, (USA), the American Institute of Physics, 1957, p. 403-420.
In FFAG betatron accelerating beam accelerators, the exciting frequency applied to the accelerator core has been limited to a conventional 100 Hz. This is due to the material used in the accelerator core. For example, although a silicon steel plate of a 100 xcexcm thickness, used in a conventional accelerator core, has a high saturation magnetic flux density, core loss and generated heat are large. Thus, operation at a high exciting frequency (1 kHz or more) is difficult.
A variation in the magnetic flux of an inner portion of the core is dependent upon the saturation magnetic flux density which, in turn, depends on the material and the cross sectional core thickness. When a core material of a high saturation magnetic flux density is used, the cross sectional core thickness may be made smaller, the (amount of) material may be reduced and the apparatus may be made smaller. However, in material of high saturation magnetic flux density, generally, core loss and generated heat are large. As a result, there is a problem in that the cross sectional thickness of the core and the size of the apparatus are increased.
In an FFAG betatron accelerating beam accelerator such as above, in a case where the exciting frequency applied to the accelerator core is 1 kHz or more, from the standpoint of temperature increase, a material of high saturation magnetic flux density and core loss must be used and there is a problem in that the size of the accelerator core is increased. On the other hand, when a small size is important and a high saturation magnetic flux density material (silicon steel plate of a 100 xcexcm or greater thickness and the like) is used, operation must be performed with an exciting frequency of less than 1 kHz applied to the accelerator core and there is a problem in that sufficient output cannot be obtained.
The present invention aims to solve the above problems and an object of the present invention is to provide a high performance beam accelerator in which accelerating voltage may be increased by making an exciting frequency applied to the accelerator core a high frequency and controlling heat generation of an accelerator core. Moreover, another object of the present invention is to provide a beam accelerator which is low cost and small in size.
According to one aspect of the present invention there is provided a beam accelerator including an annular hollow vessel formed with an annular passage inside through which passes a charged particle beam. Fixed magnetic field generating means for deflecting the charged particle beam and guiding the charged particle beam into an orbit in the annular passage is provided in plurality along a circumferential direction of the annular hollow vessel. An accelerating gap for inducing an accelerating electric field of the charged particle beam is provided at a predetermined position in the annular hollow vessel. An accelerator core for generating the accelerating electric field via the accelerating gap by changing a magnetic state of an inner portion in accordance with electromagnetic induction is provided so as to surround the annular hollow vessel.
Also, injection to ejection of charged particles is completed within one (1) cycle of an exciting frequency applied to the accelerator core.
Moreover, the accelerator core is prepared by winding in multiple layers a ribbon-shaped material of a soft magnetic alloy of 50 xcexcm or less in thickness and of a high saturation magnetic flux density of 1 T or more. Thus, core loss may be controlled and the size of the accelerator core may be reduced. Consequently, the size of the beam accelerator may be reduced and the cost may be reduced.